method has been successfully used in the design of E-plane millimeter-wave diplexers using septum type filters w13x. Morini later extended the method to the design of waveguide H-plane Y-junction multiplexers w14x.

The present work further extends Morini and Rozzi’s approach w13x and the approach in w9x by accommodating the equivalent network of the common junction in the K-inverters of the channel filters and obtains more compact diplexer designs. The paper also shows how to design each key building block of a diplexer using the most suitable analytical and numerical approach and thereby increase the overall efficiency of a design.

II. THE DIPLEXER CONFIGURATIONS

Three types of rectangular waveguide E-plane diplexers are shown in Figure 1. Figure 1a and b show the bifurcated common port diplexers without and with input transformers, respectively. Figure 1c shows the diplexer with E-plane T-junction common port diplexer. All these configurations

Figure 1. Three types of waveguide diplexers.

CAD of Wa¨eguide E-Plane Diplexers 105

can be made using a single machined conducting plate sandwiched between two identical halves of a waveguide housing. One can use either a complete circuit theory CT. or a multimode electromagnetic field modeling approach to design the diplexers. The first approach uses an equivalent circuit of channel filter K-inverters and an equivalent circuit of the common port. However, each K-inverter and the common port are synthesized using multimode field modeling. In the second approach, the overall diplexer is analyzed using multimode electromagnetic field analysis and computer based optimization. Obviously, the second approach requires more computing power and it was the one used in the 1980s. In the present work, we have used parts of both approaches in order to implement a very efficient computer aid design CAD. procedure and an improved diplexer configuration. On one hand, we have used the exact defined methods of CT synthesis of filters. On the other hand, we have used the full-wave modeling wherever required. This led to the study of two design types. The first type is based on the principle of matching a Y-type three-port w15x, which was used in w16x. The key point in this approach is the synthesis of a three-port equireflection common port. Also, each channel filter remains unchanged and their placements with respect to the port reference planes of the common junction are calculated and numerically tuned. The second approach, discussed in references w9x, uses the properties of the common junction in a different way. It uses the junction parameters as a part of the first K-inverters of the channel filters. This alters the septum widths and the lengths of the first resonators of the channel filters.

III. MATHEMATICAL MODELS

Mode Basis

The structures considered in this paper have been studied using the generalized multimode scattering matrix method. The choice of mode basis functions for the analysis of different key elements depends on the character of the exciting field, symmetry properties of the elements, and the distance between the two consecutive elements to be combined. For example, let us consider the E-plane channel filters with a common junction of E-plane bifurcation. The junction as well as the filters are homogeneous along the